The invention relates to steam turbine system overspeed protection controllers in general, and more particularly, to a system for using the stored steam energy contained in the reheater of a steam turbine system following an overspeed protection controller activation to sustain the rotating speed of the turbine at synchronous speed providing for rapid resynchronization.
A typical steam turbine system is shown in FIG. 1. A conventional steam turbine is comprised of a high pressure turbine section 10 and one or more low pressure turbine sections 12 which are generally mechanically coupled to a common shaft 14 for driving an electrical generator 16. The electrical generator 16 is used to supply electrical power to a load 18. Steam is admitted to the input of the high pressure turbine section 10 from a steam source 20 and is usually regulated by one or more governor valves 22. The steam exiting from the high pressure turbine section 10 is reheated by a reheater 24 prior to being supplied downstream to the input to the one or more low pressure turbine sections 12. One or more interceptor valves 26 may be used to interrupt the flow of steam between the input of the low pressure turbine sections 12 and the reheater 24. Steam exhausting from the one or more low pressure turbine sections 12 may be provided to a condenser 28.
The mechanical power which is developed in the high pressure and low pressure turbine sections 10 and 12, respectively, mechanically drives the electrical generator 16 which, in turn, converts the mechanical power to electrical power to be supplied to the electrical load 18. Since the coupling between the electrical generator 16 and electric load 18 is very sensitive to the frequencies of the two systems, a breaker 30 is provided to connect the electrical generator 16 to the load 18 only at times when the frequency of the electrical power generated by the generator 16 is synchronous according to a predetermined phase relationship to that of the load 18. Typically, power plant auxiliaries 32 such as electrical motors, electrical pumps, lighting, etc., are usually driven by the electrical generator 16 independent of the position of the breaker 30. Electrical power is supplied to the plant auxiliaries 32 whether the breaker 30 is open or closed to the power system load 18.
A speed/load controller 36 is generally used to govern the speed and load operation of the turbine system by controlling the position of the one or more governor valves utilizing a conventional governor valve hydraulic actuator type system 40 in accordance with measured parameters such as speed SPD, megawatt output MW, and breaker contact status BR. Examples of a speed/load controller 36 which is used for controlling the speed and load of a steam turbine system are disclosed in U.S. Pat. Nos. 3,878,401 and 3,934,128. The mechanical rotating speed of the turbine is generally monitored using a notched wheel 33, which is located on the turbine shaft 14 and rotated at the same angular velocity thereby, and a magnetic speed pickup 34 which is disposed adjacent to the periphery of the wheel 33 to supply a signal SPD representative of the turbine speed to the controller 36. In addition, a signal MW is supplied to the controller 36 from a typical megawatt transducer 38, which monitors the electrical power produced by the generator 16. And accordingly, a signal representative of the status of the breaker contacts 30 is supplied to the controller 36 over the signal line denoted as BR.
The breaker contacts 30 are also operative to disconnect the power steam turbine system from the power system load 18 at times when an electrical fault of significance is detected. It is understood that should the breaker 30 disconnect the steam turbine system from the power system load 18 at times when electrical power is being supplied thereto, the mechanical power produced by the steam turbine system will cause a mechanical overspeed to occur. For these reasons, an overspeed protection controller (OPC) 42 is provided to detect such an overspeed event and rapidly reduce the mechanical power produced by the turbine sections 10 and 12 by interrupting steam admitted thereto. Typical OPC systems are disclosed in U.S. Pat. Nos. 3,643,437; 3,826,095; and 3,826,094. This type of OPC unit (see block 42 in FIG. 1) monitors the SPD, MW, and BR signals and activates an overspeed protection control in accordance with predetermined logic conditions such as that shown in FIG. 2, for example.
Referring to FIG. 2, there exists at least two conditions which may trigger an overspeed protection control. One condition is that the SPD signal is greater than some predetermined value, normally 103% of synchronous speed. Another condition may be the interruption of the flow of electrical power from the generator 16 to the power system load 18 by opening the breaker 30 (denoted as BR) with the stipulation that the megawatts (MW) produced at the time of interruption is greater than some predetermined value, usually approximately 30%. These two conditions may be OR'ed, as shown in FIG. 2, to trigger an overspeed protection control (OPC). An overspeed protection control consists primarily of the events of energizing a number of OPC solenoids to operate hydraulic dump valves located in the governor valve and interceptor valve hydraulic actuators, 40 and 41, respectively. These dump valves when actuated operate to dump the fluid from the hydraulic actuators to drains, 44 and 46, as shown in FIG. 1, and simultaneously interrupt the hydraulic fluid supply to the governor valve and interceptor valve actuators. The governor valve 22 and interceptor valve 26 respond by immediately closing. According to the logic of FIG. 2, in order to deactivate the dump valves by deenergizing the OPC solenoids, a time delay is effected after the breaker 30 has opened which may be adjusted to some predetermined time delay interval, say 1 to 10 seconds, for example. At the end of this time delay interval should the speed be below the predetermined value typically chosen at 103% synchronous speed, the overspeed protection control will be deactivated, thereby deenergizing the OPC solenoids and causing the dump valves to no longer be in the state to dump fluid to the drains 44 and 46. During this same operation the hydraulic fluid will be resupplied to the governor valve and interceptor valve hydraulic actuators. In some systems, the interceptor valves 26 will respond to the resupply of hydraulic fluid to the hydraulic actuators by immediately reopening to its full open position. In these same systems, the governor valves 22 will remain under the control of the speed/load controller 36 after the hydraulic fluid has been resupplied to the hydraulic actuators 40. With the type of overspeed protection control described above, one might expect the turbine rotating speed to respond as that shown by the solid line curve 50 in FIG. 3 for the case when the electrical generator 16 is supplying close to 100% rated electrical power to the power system load 18 and the breaker contacts 30 are opened.
Referring to FIG. 3, the time mark t.sub.0 on the abscissa of the graph designates a point in time at which the breakers 30 of FIG. 1 are opened. Since the electrical power produced by the generator 16 just prior to the time mark t.sub.0 was assumed near rated electrical power output, an OPC activation is initiated concurrently with the opening of the breaker contacts 30. The dumping of the hydraulic fluid as a result of the OPC activation forces the governor valves 22 and interceptor valves 26 to close usually within a fraction of a second. However, as shown by curve 50 in FIG. 3, the speed is anticipated to rise beyond synchronous speed subsequent to the time mark t.sub.0 due primarily to the amount of inertia built up in the turbine system. With the interruption of steam input to the turbine sections 10 and 12, damping forces such as windage and friction losses in the turbine system cause the speed of the turbine to decay back down to some predetermined value, such as 103% which is shown at the time mark t.sub.1 in FIG. 3. The expected time interval between t.sub.0 and t.sub.1 is on the order of 50 to 60 seconds, but may vary from turbine to turbine.
At the time t.sub.1, the OPC signal is deactivated in accordance with the logic shown in FIG. 2 thus allowing for the interceptor valves 26 to be operated to their wide open position and the steam which has been stored in the reheater 24 during the OPC activation is admitted through the interceptor valves 26 to the low pressure turbine sections 12. The rotating speed of the turbine is then again increased greater than the 103% synchronous speed value which causes another activation of the overspeed protection control as controlled by the logic of FIG. 2. These activations and deactivations of the overspeed protection control will continue to occur, see times t.sub.2, t.sub.3 and t.sub.4 of FIG. 3 until substantial amount of the steam energy has been dissipated from the reheater 24. A typical dissipation curve is shown by the dashed line 52 in FIG. 3. It has been estimated that the number of speed oscillations shown typically between the time intervals depicted in the graph of FIG. 3 may amount to as many as 10 or 12 over a time period of approximately 10 to 12 minutes.
In the types of OPC systems just described, it is unlikely that resynchronization of the turbine system to the load can occur until the frequency oscillations of FIG. 3 have stopped. It is evident then, in order to have rapid resynchronization, these oscillations should be eliminated while still providing overspeed protection to the turbine system. An overspeed protection controller which could provide a rotating speed response curve such as that depicted by the dotted line 54 in FIG. 3 is desired. In this example, protection against overspeed is provided immediately following the opening of the breaker 30 at time t.sub.0, but at time t.sub.1 no reactivation of the overspeed protection control is performed and speed is thereafter controlled at a synchronous speed value. If the rotating speed could be controlled in this manner, resynchronization to the power system load could be performed then at any time subsequent to t.sub.1. Even for the case when resynchronization is not required, the electrical power supply to the plant auxiliaries 32 will be maintained at a near fixed frequency level after the frequency excursion between times t.sub.0 and t.sub.1 as a result of the opening of breaker contacts 30.